Preheat your oven to 350°F (175°C). In a medium bowl, mix together the graham cracker crumbs, granulated sugar, and melted butter until well combined.
Press the graham cracker mixture firmly into the bottom of an 8x8 inch baking dish to form the crust. Bake for about 10 minutes or until lightly golden. Remove from the oven and let cool.
In a separate bowl, whisk together the sweetened condensed milk, key lime juice, egg yolks, and key lime zest until smooth and well combined.
Pour the key lime mixture over the cooled crust, spreading it evenly.
Bake in the oven for an additional 15-20 minutes, until the filling is set and slightly firm to the touch.
Once done, remove from the oven and let it cool to room temperature. Then, refrigerate for at least 2 hours before cutting into bars.
Cut into squares or rectangles and serve chilled, topped with a dollop of whipped cream and a slice of lime if desired.
Notes
Chill for at least 2 hours before serving for best texture.